Diamond semiconductor devices attract attention as devices which would work stably at high temperature or in rigorous surroundings, e.g. under the radiation of radioactive rays or devices which could generate high output power.
These properties of diamond semiconductor devices are attributable to thermal or chemical stability of diamond. Since pn junctions of diamond semiconductor are difficult to fabricate, Schottky junctions are often used instead of the pn junctions. Therefore, the formation of the Schottky junctions with ideal properties is one of the most important objects for the diamond semiconductor devices.
The quality of a Schottky junction is estimated by the diode properties. Namely, a good Schottky junction requires a small reverse current for a reverse bias voltage and the small voltage drop for the forward bias. Some trials for fabricating a Schottky junction of semiconductor diamond have been done.
M. W. Geis et al have reported a point contact type of a Schottky junction of diamond semiconductor; M. W. Geis et al, IEEE ELECTRON DEVICE LETTERS, vol. EDL8, (1987) p.341.
However, this Schottky junction had such a small contact area that it was inapplicable to high output power devices. Although the research of metal electrodes formed by evaporation coating has proceeded to improve the quality of Schottky junctions of diamond semiconductor, the method for fabricating a Schottky junction of good quality has not been established yet.
To estimate the quality of a Schottky junction, the reverse current calculated from the height of a barrier, and the n-value which appears in the denominator of an exponential function describing the relation between the applied voltage and the forward current are often used.
Here, the reverse current includes not only the ideal reverse current overflowing the Schottky barrier or flowing through the Schottky barrier by the quantum tunneling effect, but also the leakage current flowing due to the insufficiency of the Schottky junction or flowing by hopping on the defect levels existing in the band gap. The quality of a Schottky junction is estimated by the strength of the leakage current, because the leakage current depends on the insufficiencies or defects of the junction.
In the case of a Schottky junction between metal and diamond, if the height of the Schottky barrier between metal and semiconductor diamond is assumed to be about 1.8 eV, the ideal reverse current density (without leakage current) should be about 10.sup.-23 A/cm.sup.2.
However, in practice, there are some leakage currents, which raises the reverse current density far above 10.sup.-23 A/cm.sup.2. Current detectors do not have sensitivity sufficient to detect a small current density, e.g., less than 10.sup.-12 A/cm.sup.2. If the leakage current density is more than the limit of detector sensitivity of 10.sup.-12 A/cm.sup.2, the quality of the Schottky diode can be estimated by the actual leakage current, if the leakage current is smaller than 10.sup.-12 A/cm.sup.2 ; the marginal sensitivity of current detectors, and thus quality of the Schottky diode cannot be measured in a quantitative way but rather the quality is objectively considerably good.
Theoretically, the forward current shall be expressed by exp( eV/nkT ), where e is the charge of an electron, k is the Boltzmann constant, T is the absolute temperature and V is the voltage applied to the Schottky junction. The "n" in the denominator of the index is the n-value mentioned. In the case of an ideal Schottky junction, n=1.
It was reported that the point contact type of Schottky junction had exhibited the reverse current density of 10.sup.-9 A/cm.sup.2 to 10.sup.-6 A/cm.sup.2 and the n-value of 2 to 10. The reverse current density was far bigger than the ideal reverse current density of 10.sup.-12 A/cm.sup.2 (in the case of the barrier height of 1.8 eV) and also still bigger than the marginal current density of 10.sup.-12 A/cm.sup.2 which a current detector can sense. Furthermore, the n-value differs from the ideal value to a degree.
The bad quality of the point contact type of Schottky junction of diamond semiconductor proceeds from the bad surface state of the diamond semiconductor. The properties of Schottky junctions are heavily affected by the surface state of the semiconductor. Since the Schottky junction is fabricated by evaporating a metal layer on a semiconductor diamond layer, not only the deposition of the metal layer but also the surface state of the semiconductor diamond layer are important factors deserving of full consideration.
The carbon atoms on the surface of diamond have some extra bonds which extend upward. In the case of the diamond which was synthesized by the CVD method, the extra bonds of the carbon atoms couple with the bonds of the neighboring carbon atoms or the bonds of hydrogen atoms. Thus, neighboring two carbon atoms are tightly coupled. The coupled carbon atoms are aligned in a periodic two-dimensional order on the surface. The periodicity of the surface carbon atoms is twice as long as that of the periodicity of crystal. Although a carbon atom on the surface has two neighboring carbon atoms, it couples with one of the two atoms by the extra bonds. The coupling by the extra bonds brings about an anisotropy on the surface structure. If metal electrodes are deposited on the diamond surface to make a Schottky junction, the quality of the Schottky junction is not good because of the bad surface state of the diamond.
A purpose of this invention is to provide a Schottky junction of diamond semiconductor with low reverse current less than 10.sup.-13 A/cm.sup.2 ( the limit of sensitivity of current detectors) and with an n-value of 1 to 2.